Using NMR Techniques to Find Partition Coefficients Across Biphasic Systems

A model biphasic system was studied using acetone on 1-octanol and water. The partition coefficient was found using z-axis single pulse slice-selective spatially resolved excitation (SPS3RE) nuclear magnetic resonance (NMR) spectroscopy. This SPS3RE NMR experiment allows for the collection of a one-dimensional look at 1H spectra at defined spatial intervals along the z-axis, providing analyte quantitation through the entire sample. With this in mind, the concentration of acetone was characterized as a function of spatial location from the two-phase interface leading to the determination of partition coefficients by signal integration. This system was then expanded to analyze the hydrolysis of parathion in a biphasic system of parathion and deuterated sodium hydroxide with varying amounts of tert-butylammonium chloride. From this, the utility of a simplified, slice-selective pulse for analyzing heterogeneous samples is observed

Parathion Hydrolysis Revisited: In Situ Aqueous Kinetics by \u3csup\u3e1\u3c/sup\u3e H NMR

The kinetics of parathion (PTH) decomposition into para-nitrophenolate (pNP) and O,O-diethylthiophosphate (DETP) were measured in high-pH aqueous solutions at 20 °C by proton nuclear magnetic resonance spectroscopy (1H NMR). Reaction rates were determined over a 16 h observation time, in solutions with NaOD concentrations of 5.33 mM, 33.33 mM, and 100 mM, with NaCl added to fix ionicity. The pseudo-first-order rate constants for these systems were determined to be 1.9 × 10–4 min–1, 1.4 × 10–3 min–1, and 3.8 × 10–3 min–1 respectively. The slope of the linear plot of these rates against OD– concentration yielded the second-order hydrolysis rate constant 3.90 × 10–5 mM–1 min–1, valid over this pH range from 10.5 to 13. The data agree with some, and contradict other, earlier work. Our fitting procedure included background levels and allowed us to not only obtain reliable kinetic results but also to measure residual pNP and DETP impurity levels

Origin and Correction of Magnetic Field Inhomogeneity at the Interface in Biphasic NMR Samples

The use of susceptibility matching to minimize spectral distortion of biphasic samples layered in a standard 5 mm NMR tube is described. The approach uses magic angle spinning (MAS) to first extract chemical shift differences by suppressing bulk magnetization. Then, using biphasic coaxial samples, magnetic susceptibilities are matched by titration with a paramagnetic salt. The matched phases are then layered in a standard NMR tube where they can be shimmed and examined. Linewidths of two distinct spectral lines, selected to characterize homogeneity in each phase, are simultaneously optimized. Two-dimensional distortion-free, slice-resolved spectra of an octanol/water system illustrate the method. These data are obtained using a 2D stepped-gradient pulse sequence devised for this application. Advantages of this sequence over slice-selective methods are that acquisition efficiency is increased and processing requires only conventional software

VARIABLE-ANGLE 3-DIMENSIONAL EXCHANGE NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY FOR THE STUDY OF MOLECULAR-MOTION IN COMPLEX SOLIDS

Although molecular motions are responsible for many of the macroscopic properties observed in solids, especially in polymers, methods for studying these processes in all but the simplest systems are scarce. In the present study we introduce a three-dimensional nuclear magnetic resonance experiment for characterizing ultraslow molecular motions in complex solid systems. The technique extracts dynamic information by resolving the two-dimensional exchange distributions that can be observed in spectra of static samples, according to the isotropic chemical shifts of individual molecular sites. These three-dimensional correlations are achieved by processing signals arising from a fast-spinning solid sample using two independent macroscopic axes of rotation as extraction parameters, an approach which becomes practical due to the simple scaling behavior of anisotropic chemical shifts with respect to the axis of sample rotation. The principles involved in this new spectroscopic technique are discussed, and the method is illustrated with an application to the analysis of motions in isotactic polypropylene

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Nuclear magnetic resonance imaging (NMRI) holds the potential for the non-destructive evaluation of ceramics and for the improvement of ceramic processing in general. It can provide valuable diagnostic information about the spatial variations of binders, plasticizers, sintering aids, deflocculants, and other organics in injection-molded and slip-cast green ceramics. Poor distribution of these organics, after subsequent processing steps such as sintering, hot isostatic pressing, and machining, can lead to final parts that are defective and/or with poor mechanical propertie

Rotational diffusion measurements of suspended colloidal particles using two-dimensional exchange nuclear magnetic resonance

We present here an experimental and theoretical study of the application of two-dimensional exchange nuclear magnetic resonance spectroscopy ~NMR! to the investigation of the rotational diffusion of colloidal particles. The theoretical discussion includes the nature of the NMR frequency time-correlation function where the NMR interaction is represented by the chemical shift anisotropy ~CSA!. Time-correlation functions for the isotropic rotational diffusion of a suspension of colloidal particles containing single and multiple sites are derived in addition to time-correlation functions for the rotational diffusion of a suspension of symmetric top particles containing an isotropic distribution of a single CSA interaction. Simulations of two-dimensional exchange spectra for particles undergoing isotropic rotational diffusion are presented. We performed two-dimensional exchange NMR experiments on a colloidal suspension of spherical poly~methyl methacrylate! ~PMMA! particles which were synthesized with a 20% enrichment in 13C at the carbonyl site. Rotational diffusion time-correlation functions determined from the experimental exchange spectra are consistent with the composition of the colloidal suspension. Detailed explanations of the syntheses of the enriched methyl 13C-~carbonyl!-methacrylate monomer and the small quantities of 20% enriched 13C-~carbonyl!-poly~methyl methacrylate! microspheres used for this study are presented

Examining the Impact of Steric and Electronic Variation in N\u3csub\u3e2\u3c/sub\u3eS Scorpionate Ligands on the Properties of Zinc(II) and Cadmium(II) Complexes

A series of LZn(II)Br (1–4) and LCd(II)Cl complexes (9–11) has been prepared by the reaction of metal halide precursors with the lithium salts of the N2S− ligands bis(3,5-diisopropylpyrazol-1-yl)dithioacetate (L1), bis(3,5-di-tert-butylpyrazol-1-yl)dithioacetate (L2), N-phenyl-2,2-bis(3,5-diisopropylpyrazol-1-yl)thioacetamide (L3) and N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide (L4). Characterization by X-ray crystallography and DOSY NMR studies indicate that LZnBr complexes 1–4 are mononuclear both in the solid state and in solution. Steric differences between ligands L1–L4 result in distortion from an ideal tetrahedral geometry for each complex, with the degree of distortion depending on the bulk of the ligand substituents. In contrast, the related complex L3CdCl was shown by X-ray crystallography to dimerize in the solid state to form the chloride-bridged five-coordinate complex [L3CdCl]2 (10). Despite 10 having a dinuclear structure in the solid state, DOSY NMR studies indicate 9–11 exist as mononuclear LCdCl species in solution. In addition, Zn(II) cyanide complexes of the form LZnCN [L = L1 (5), L3 (7), L4 (8)] have been characterized and the X-ray structure of 8 determined. Moreover, density functional theory calculations have been conducted which yield important insight into the bonding in 1–4 and 5–8 and the electronic impact of ligands L1–L4 on the zinc(II) ion and its ability to function as a Lewis acid catalyst